The SU carburettor is at the heart of the Midget's fuel system, and correct identification of the carburettor variant is the single most important step when ordering any service or replacement component. The 1275cc and 1500cc use different carburettor sizes with entirely different internal components, and within each engine type there are further sub-variants that affect needle, jet, and gasket specification. Each carburettor carries an aluminium tag attached to one of the float chamber lid screws stamped with the variant number, suffixed 'F' (front) or 'R' (rear).
1275cc SU HS2 Carburettors
The 1275cc Midget was fitted with twin SU HS2 carburettors throughout its production, with a 1¼-inch bore. Three home-market variants were used. The earliest variant fitted to the 12CC engine is non-ventilated with no breather hose union on the body and uses a fixed needle. The mid-production variant fitted to the 12CE, 12V586F, and 12V588F engines is ventilated with a breather union on the body and uses a fixed needle.
The final variant fitted to the 12V778F engine is ventilated with a breather union and uses a biased (sprung) needle. Where carburettor tags are missing or illegible, these physical features identify the variant.
1275cc Service Components
Key service components include the float and needle-and-seat assembly (common across all HS2 variants), the dashpot piston damper, dashpot spring (blue for the HS2), throttle spindle kits, throttle disc kits, and gasket kits. The jets differ between front and rear carburettors. Service kits and complete rebuild kits are available, the rebuild kit contains the float, jets, needle-and-seat, and gasket kit as a comprehensive package.
1275cc Complete Carburettors
Complete new and reconditioned carburettor pairs are available for each of the three home-market HS2 specifications. The carburettor variant must be matched to the engine, as the breather arrangement and needle type are integral to the engine's fuelling specification. Reconditioned units are supplied on an exchange basis with the customer's original carburettors returned for refund of the exchange deposit.
1500 SU HS4 Carburettors
The 1500 used twin SU HS4 carburettors with a 1½-inch bore, physically larger than the HS2 and with correspondingly larger internal components. Four production variants were fitted across 1500 production.
The first two variants are identified by a flat damper piston cap, with the earlier specification using a thin jet body and the later using a wide jet body. The two later variants are identified by a raised damper piston cap with the jet assembly part identification moulded into the carburettor body itself.
1500 Service Components
The dashpot piston spring on the HS4 is red, distinguishing it from the blue spring used in the HS2. The damper, jet assemblies, and needle guides differ between the early and late HS4 specifications. Jet assemblies for the front and rear carburettors have different part numbers within each specification. Service kits and rebuild kits are specific to the carburettor tag number, so this must be quoted when ordering.
1500 Complete Carburettors
Complete new and reconditioned carburettor pairs are available for each of the four HS4 specifications. As with the HS2, the variant must be matched to the engine, and reconditioned units are supplied on an exchange basis.
SU Tuning Tools
A specialist SU carburettor tuning tool kit is available for checking suction chamber free movement, synchronising twin installations, adjusting mixture, centralising jets, and setting float chamber fuel levels. These tools are invaluable for both routine servicing and for recalibrating carburettors after fitting performance air filters or exhaust modifications.